Autodeposition coatings, which are adherent coatings formed on metal surfaces, comprise an organic polymer coating deposited by electroless chemical reaction of the coating bath with the metal surfaces. Autodeposition has been in commercial use on ferrous surfaces, in particular steel surfaces, for about thirty years and is now well established for that use. For details, see for example, U.S. Pat. No. 3,592,699 (Steinbrecher et al.); U.S. Pat. Nos. 4,108,817 and 4,178,400 (both to Lochel); U.S. Pat. No. 4,180,603 (Howell. Jr.); U.S. Pat. Nos. 4,242,379 and 4,243,704 (both to Hall et al.); U.S. Pat. No. 4,289,826 (Howell, Jr.); and U.S. Pat. No. 5,342,694 (Ahmed) as well as U.S. Pat. No. 5,500,460 (Ahmed et al.). The disclosures of all of these patents are hereby incorporated by reference. Additional compositions and processes for depositing autodeposition coatings are described in U.S. Pat. Nos. 6,989,411; 6,645,633; 6,559,204; 6,096,806; and 5,300,323, incorporated herein by reference.
Autodeposition compositions are usually in the form of liquid, usually aqueous, solutions, emulsions or dispersions in which active metal surfaces of inserted objects are coated with an adherent resin or polymer film that increases in thickness the longer the metal object remains in the bath, even though the liquid is stable for a long time against spontaneous precipitation or flocculation of any resin or polymer, in the absence of contact with active metal. The resin or polymer used in autodeposition baths is desirably insoluble in water. “Active metal” is defined as metal that is more active than hydrogen in the electromotive series, i.e., that spontaneously begins to dissolve at a substantial rate (with accompanying evolution of hydrogen gas) when introduced into the liquid autodeposition solution, emulsion or dispersion. Typically, the working autodeposition baths are acidic in nature, having pHs ranging from about 1 to about 4. Such compositions, and processes of forming a coating on a metal surface using such compositions, are commonly denoted in the art, and in this specification, as “autodeposition” or “autodepositing” compositions, dispersions, emulsions, suspensions, baths, solutions, processes, methods, or a like term.
Generally, after removal from the autodeposition bath, the autodeposited coating undergoes a rinse step and a cure step prior to the addition of any paint layer. That is, the autodeposition coating is dried and fully cross-linked before addition of another paint. In a typical processing line for depositing a layer of autodeposition coating and a subsequent layer of paint, two ovens are required; the first oven is used for curing (cross-linking) the autodeposition coating. After exiting the first oven, powder or liquid paint, for example a topcoat, is applied and the part enters a second oven for cure of this second layer, e.g. topcoat, of paint.
The cure step for the autodeposition coating conventionally takes place at temperatures of 165-204° C. for a duration of 10 to 30 minutes. These temperatures have been thought to be required to achieve adequate cross-linking of the autodeposition coating, and use of this cure step to likewise cure secondary paint layers was not considered possible due to gases exiting the autodeposition coating during curing. Prior art attempts to do so resulted in paint defects in the second paint layer surface. As such, after cure of an autodeposition coating in a conventional process, the coated part would be subjected to a second painting step and a second cure step for the paint. Similarly, electrodeposited coating is formulated with low molecular weight epoxy and higher loading of blocked isocyanate and other byproducts (amine and solvent) which requires the electrodeposited coating to be fully cured prior to topcoat application as exiting byproducts cause surface defects. To improve manufacturing efficiencies, it is desirable to combine the above described two curing steps into a single step.
Advantages of co-curing include eliminating processing steps, reducing oven length and reducing oven temperature. It is also desirable to have an uncross-linked autodeposition coating that is dry to handle which allows for transient time, such as for example where a topcoat is to be applied at a location different from the autodeposition coating.